In September 2007 NASA launched the Dawn space probe. While earlier multi-target missions such as Voyager could only do flybys, Dawn was the first NASA mission to use ion propulsion, which has allowed it to enter and leave the orbits of multiple objects in space. After exploring the asteroid Vesta, starting in July 2011, it began orbiting the dwarf planet Ceres in March 2015.

While images taken by Dawn show the surface of Ceres to be generally quite dark, they also show more than 300 mysterious bright spots known as faculae. Where Ceres was previously thought to be a dead world, these reflective areas have been found to change over time, suggesting an active, evolving environment.

In a recent study in the journal Icarus lead author Nathan Stein of Caltech divides the bright spots into four categories. One group contains the most reflective material on Ceres, found on crater floors. These bright areas consist of salt-rich material that most likely was once mixed with water. While the faculae stand out against the planet’s generally dark surface, even the brightest of these, Cerealia Facula, which covers a six mile wide area, would look to us like dirty snow.

A more common type of bright material is found on the rims of craters, streaking down towards the crater bases. Celestial bodies striking Ceres most likely exposed bright material that was already present below the planet’s surface or that had formed during previous impacts. Billions of years ago, during a time when Ceres experienced far more impacts, its surface would have had many more such bright spots.

A third type of facula is found where material was ejected when craters were formed.

And then there’s Mount Ahuna Mons, a mountain that has its own category, the only known case in which bright material is not associated with an impact crater. Most likely a cryovolcano formed by slowly flowing icy material, it has bright streaks down its sides.

One puzzle is why the bright areas in Ceres’ Occator Crater seem so different from one another. Dr. Lynnae Quick, a planetary geologist at the Smithsonian Institution in Washington, suggests that Occator Crater could have had a reservoir of salty water beneath it in the recent past. Vinalia Faculae, the diffuse bright areas near the crater’s dome, could have formed from liquid propelled to the surface by small amounts of subsurface gas. The dissolved gas is thought to have been a volatile substance such as water vapor, carbon dioxide, ammonia or methane. Lower pressure at the planet’s surface would have caused the liquid to boil off as vapor.

Cerealia Facula, also in Occator Crater, most likely formed in a different way, given that it is brighter and more elevated than Vinalia Faculae. The material there is thought to have been icy lava that seeped through fractures in Ceres’ surface to form a dome. Intermittent periods of boiling, similar to those that occurred as Vinalia Faculae formed, may have happened here too, leaving a residue of ice and salt particles to form the Cerealia bright spot.

The fractures that have allowed for the formation of these bright spots probably resulted from the impact of the large object that crashed into Ceres 34 million years ago, creating the 57 mile wide Occator Crater. The bright material in the crater is about 4 million years old, and it is thought that the process that created it is still ongoing on Ceres, as evidenced by a thin haze, still present today, most likely due to sublimating ice. This haze would soon disappear into the vacuum of space if the icy materials that caused it were not being replenished.

As the Dawn probe orbits closer to Ceres in the final stage of its mission, it is hoped that more will be discovered about the origins and nature of the dwarf planet’s bright spots.

Join the Springfield Stars Club on Tuesday, January 23rd at 7:00pm at the Springfield Science Museum for “Open Mic Night,” chaired by club president Alan Rifkin. Come and share your love of astronomy and your astronomy experiences. Refreshments will be served, and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per non-member.

Also, on Friday, February 2nd at 7:30pm, the Stars Club and the Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. Astronomy educator and Naturalist Club president Dave Gallup will host the “Oceans in Space” planetarium show. A fee of $3 for adults and $2 for children under 18 will be charged.